DE 31 32 429 C3 relates to an extruder screw of the above-indicated kind, wherein the screw has an intake zone, a transformation zone and an ejection zone. The screw is of a dual-flight nature in the intake zone and in the initial region of the transformation zone. Disposed in the first third of the transformation zone are installation fitments in the form of pins fixed on the periphery of the screw core, in a row arranged at a right angle to the axis of the screw between the screw lands. That enhances the mixing effect within the molten plastic material.
A single-screw extruder for continuously processing thermoplastic materials having a screw which is arranged rotatably in a housing and which has a shearing and a mixing portion is known for example from DE 42 27 018 C2. The shearing and mixing portion is provided in a portion of the extruder screw, that is free from any screw land.
The known shearing and mixing portions are generally of such a nature that a mixing effect of the utmost intensiveness is achieved by installation fitments protruding into the screw flight, over a shortest possible portion of the extruder screw.
That leads to a great rise in pressure in the mixing region in question of the extruder screw, whereby an over-proportionally high level of frictional heat is introduced into the extrudate in a comparatively short portion of the extruder screw. That leads to a local increase in the material temperature, which is often undesirable. Usually, screw extruders for plasticising and transporting thermoplastic material are heated by what are referred to as heating belts, but nonetheless a considerable part of the plasticisation energy is introduced into the extrudate by way of shearing of the material. It is therefore desirable to keep the input of frictional heat into the extrudate within reasonable limits.
In addition the mixing portions or mixing parts in accordance with the state of the art suffer from the disadvantage that they represent obstacles to the through-flow of material, which adversely affect the through-put capacity of the extruder.